The invention relates to tire treads, and in particular to a structure of a new incision geometry, imparting improved properties of adherence to said treads while postponing the occurrence of irregular wear.
The invention relates to treads intended for the fabrication of new tires or to the retreading of tires, and in particular to tread structures for said tires, comprising a large number of cut-outs in the form of grooves and/or incisions. By incision is meant, in this description, a cut-out of width less than or equal to 2 mm, while a groove is a cut-out of width greater than 2 mm.
It is known, for example in U.S. Pat. No. 2,100,084, that incisions may be made whose trace on the tread surface of a tire is of closed circular shape, providing active edges in all directions tangent to the tread surface. Each such incision bounds and isolates a rubber element (of small size compared to the tread width) joined to the tread at its base only. This rubber element, under the action of forces of rolling contact with the ground, has a tendency to deform both in shear and in bending (stresses tangential to the surface of the isolated element coming into contact with the ground) and in compression under the load supported by the tire. Considering the size of the isolated elements and of the stresses to which they are subjected, it may happen that these elements are torn away from the rolling tread.
To remedy this drawback, it is known, for example in U.S. Pat. No. 2,938,560, that the walls of the isolated element may be provided with rubber connecting elements joining said element to the rest of the tread. U.S. Pat. No. 2,938,560 describes a tread provided with a structure made up of a plurality of wave-shaped incisions, said incisions being arranged under as to isolate rubber studs; these studs are joined to the tread by four supports arranged on the tread surface, each support rising beneath the rolling surface in new condition and extending in a direction perpendicular to said surface, as far as the full depth of said incisions. By stud is meant in the description a small isolate rubber element.
However, such incisions may lead to irregular wear, that is, wear occurring preferentially in certain areas of the tread and amplified in rolling so as to lead usually to premature discard of the tire; in some cases, the edges formed by the incisions in the tread may be preferential locations of initiation of such wear, especially in applications having a large tread thickness (for example in tires for heavy loads).
Furthermore, the incisions proposed in U.S. Pat. No. 2,938,560 have another disadvantage relating to the entry of pebbles into the tread.
It often happens that the ground on which the tire rolls is covered with gravel or other objects that may get into the incisions. In many cases, these introduced objects are ejected under the action of centrifugal force due to rotation of the tire; all the same, it will happen that some of these foreign bodies remain imprisoned in said incisions and may travel towards the bottom of the incisions until they attack the tire structure. It is likewise possible that these included objects may cut the rubber studs bounded by each closed-curve incision, with the result that the rubber will tear, forming holes in the tread and thus increasing the likelihood of penetration by foreign bodies.